Material having a thermally expandable passage

ABSTRACT

A bilayer laminated material has a gas permeability increasing greatly with temperature, achieved by cutting a star-shaped hole in it when it is relaxed and flat. On warming, the leaves between the radial cuts curl and disproportionately increase the hole size. Such a material, if flat at 5° C. and wide open at 20° C., is useful for packaging vegetable produce.

BACKGROUND OF THE INVENTION

The present invention relates to a material having a passagetherethrough. (The passage may be completely closable.) Such materialhas a wide variety of applications. For illustration, the invention willbe described with reference to one such application, namely packaging ofedible produce, in particular to maintain it in a preservativeatmosphere, bearing in mind that the requirements can vary withtemperature.

All harvested vegetables, flowers and fruits continue to respire,absorbing oxygen and emitting carbon dioxide. A number of parallelbiological paths are involved leading to loss of weight (although lossof water is usually the main cause of weight loss), some heatproduction, loss or change of flavour, change of texture, anddiscoloration of outer surfaces or cut surfaces or in the body of theproduct. The growth and excretions of micro-organisms present atharvesting can produce other changes. Virtually all these changes aredeleterious to the acceptability of the product to the consumer.

Respiration increases with increasing temperature. The main method ofincreasing storage life is therefore to hold the product at loweredtemperatures, often a little above 0° C. However some products (e.g.runner beans, cucumbers, green peppers, tomatoes, bananas) cannot bestored below 5°-11° C. Respiration rates show a wide range, broccolibeing nearly 10² times as fast as onions or potatoes. Values also varywith variety within a given species, cropping and harvesting methodsused, year of crop and degree of ripeness.

It might be thought that the respiration rate (in particular the carbondioxide production rate) could be sharply reduced by lowering theconcentration of oxygen. However below 2-3% (depending on product)anaerobic processes begin, due to both the product and its accompanyingmicro-organisms. These, usually, produce rapid and unacceptable taintingof flavour. Alcohol production by yeasts often predominates whilealdehydes are often the main cause of unacceptability.

If a product is wrapped in a sealed impermeable packaging system theneven a modest respirer such as tomatoes soon lowers the oxygenconcentration to anaerobic conditions. Ripening of tomatoes iscompletely inhibited and does not resume when the package is opened andmany of the fruits develop rots or suffer fungal attack. (An atmosphereof 6% oxygen retards ripening, which however recommences on opening toair, with no adverse effects on eating quality.) To overcome this rotproblem, it is known to pass such a sealed impermeable pack (preferablyafter a short period of storage and temperature reduction, to allow theoxygen concentration to drop rapidly), under an adjustable mechanical`pecking` head which punches in the necessary holes, e.g. typicallythree holes of0.4 mm² per pack. Changing their diameter or numberprovides easy adjustment for product type, variety type, weight andexpected downstream conditions before consumption etc--always providedthe basic knowledge is available. However, by itself, this has beenfound to give inadequate variation with temperature.

An alternative approach to this problem has been to devise selectivelypermeable materials which can be used as panels in gas-impermeablewrapping materials for fresh fruit, vegetables or flowers. These panelscan be of a microporous material having greater CO₂ permeability than O₂permeability, and it has been proposed to use that area of material aswill transmit oxygen at the same rate as the packaged contents willconsume it. However, this presupposes a certain temperature of storage,as oxygen consumption of produce increases much more sharply withtemperature than does the oxygen transmissibility of the material. Ataccidentally high temperatures, therefore, the oxygen within such awrapping will be consumed much faster than it can be replenished,leading to anaerobic conditions and their drawbacks as alreadydescribed.

It is known that high carbon dioxide concentrations inhibitmicro-organisms so that it could be advantageous to maintain such levelsfor packaged fruit and vegetables which tolerate such concentrations.

Regarding water vapour, the optimum requirement seems to be a relativehumidity of just under 100%, but with no liquid water present, both fromappearance of the pack and for the discouragement of fungal growths.

It would therefore be desirable to devise a material having a passagetherethrough, which passage changes in size with temperature at a higherrate than the thermal coefficient of expansion of the material, over atleast a certain temperature range, e.g. increasing as temperature risesfrom -5° C. or 0° C. to 20° C.

SUMMARY OF THE INVENTION

According to the present invention, a flexible material isasymmetrically laminated from at least two layers having differentcoefficients of thermal expansion, the material having a non-straightslit, at least one and preferably at least two layers (preferablyadjacent) being of plastics. At least one layer may be a metal foiladhered to a plastics layer or may be a layer of metal depositeddirectly thereonto, by any suitable metallisation method. Note that thetwo layers contribute synergistically to the variation of aperture sizewith temperature. The asymmetry is such that the material tends to curlas its temperature moves away from a so-called lie-flat point. Thelie-flat point preferably is within the range -5° to 20° C., morepreferably within the range 0° C. to 20° C. The lie-flat point is often(but not always) the temperature of the layers at the time they werelaminated. The asymmetry can thus reside in the identities of the layersor in their respective thicknesses or in both. As to thicknesses, it ispreferred that the layers (preferably all, but most or some will do,especially if it is the two outside ones) of a material have thicknessesin proportion to the inverse cube root of their respective Young'smoduli, to within a reasonable approximation.

In one aspect, the slit has the form of two or more slots radiating froma junction, at least one pair of adjacent slots preferably forming anangle of at most 90° (and more preferably at least two pairs of adjacentslots form an angle of at most 60°). The junction may be an aperture ofsensible size. Some or all of the slit may be produced by laser.

The aperture may be constant or adjustable ranging from one hole (whichwould typically be from 0.05 to 1.0 mm² for most retail packs andproducts), to many holes of few microns diameter.

The gas permeability of the apertured laminate should increase at leastthreefold from 5° C. to 20° C. This would make it especially suitable tobe used in a package (e.g. comprising all or part of the wrapping) forrespiring edible produce.

A packaging film has to fulfill many demands, e.g. of strength,toughness, often clarity, or transparency sealability, printability,ability of the inner surface to disguise or spread water droplets, andfinally price. To achieve this while offering a controlled and specificseries of permeabilities with a prescribed temperature variation is tosteer between Scylla and Charybdis while gazing on Medusa. Therefore toinstances where a material according to the invention cannot be used asthe packaging film itself, there is merit in the concept of a small areaof material according to the invention in a package or other containermade otherwise from a conventional commercial packaging film. The smallarea needs no transparency etc, only a suitable permeability andstrength.

The invention may be realised for packaging edible produce and otheruses (exemplified later) by films or membranes of plastics, polymers orother materials, and/or of stouter sheets such as of metal foil or sheetand/or of semi-rigid sheets of plastics or polymers, which themselvesmay be loaded with metal powder. The materials are preferably inert(especially, dimensionally inert) to moisture, in particular do notswell when humid or wet, and may indeed be hydrophobic. Two-layerlaminates are preferred, especially where one layer is polyester and/orthe other is polyolefin (usually polyethylene). Alternatively,polyamide, cellulosic or polyethylene terephthalate film can be pairedwith low density polyolefin (e.g. polyethylene) film, which may bemetallised, or with ethyl vinyl acetate film. It will be self-evidentthat the laminate must be asymmetrical; thus a symmetrically laminatedmaterial of layers of polymers A, B, C, B, A where the A thicknesseswere equal to each other and the B thicknesses were equal to each otherwould be thermally inert. The thermal coefficients of expansion of thematerials should be very different.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, in which FIG. 1 shows in cross section andFIG. 2 in plan a material according to the invention, having atemperature dependent variable-shape hole. FIG. 3 shows in plan amaterial according to the invention with different apertures. FIG. 4shows in cross-section one of the apertures of FIG. 3, taken on a planethrough the vertex of that aperture (the V-shaped one). FIGS. 5 and 6show two version of a pre-pecked stick-on patch for applying to a 1 cmsquare hole in a product package.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Consider FIGS. 1 and 2. The system uses a two layer film, of twopolymers with fairly widely different temperature coefficients ofexpansion, for example polyethylene and polyester. Each film is madeseparately by blowing or calendering and annealing for flatness, andthen the films are stuck together by a thin layer of flexibleelastomeric adhesive such as neoprene while held flat at a temperatureT₁ (e.g. 0° C.) being the lowest storage temperature required for theproduct. An alternative adhesive is a thin layer of an aqueous emulsionof a modified acrylic adhesive such as 3M (trade mark) 3565.Solvent-based adhesives can be used for strength. Another successfultwo-layer film pairing is 12-micron biaxially oriented polyethyleneterephthalate film stuck to 36-micron low-density polyethylene filmplasma-treated on one face to improve adhesion with a 12 micron (wet)(=6-8 micron when dry) layer of 3M 3565 adhesive, the whole consolidatedby a rubber roller. In general, as remarked previously, it is preferredthat the lie-flat temperature should be somewhere in the range -5° to20° C., usually 0° to 20° C. Sample films made at 4° C., 8° C. and 20°C. respectively all curled up to form a roll (polyethylene outwards) ofunder 1 cm diameter when subjected to a 20° C. temperature rise. In thisfilm a "pecked hole" is made of a suitable size (e.g. 1/4 mm²) for T₁.Radiating at equal angles apart from the hole are four (possibly 6 or 8)cuts each 3 mm long in the film. As the temperature rises from T₁ to T₂the differential expansion of the two film components causes the radialsegments between the cuts to curl, as in a bimetallic strip, andincreases the area of the total void (i.e. both the hole itself andopening of the radial cuts). As an alternative to a pecked hole, amaterial of high inherent permeability could be used; for example, a 25micron film of ethylene-vinyl acetate transmits 12 litres of oxygen perm² per day per atmosphere overpressure at 23° C. at 0% relativehumidity.

From the foregoing, it will be seen that there are two possibilities ofincorporating such holes. First is to use the bicomponent film for thewhole pack. (Already multicomponent packaging film is commonplace. Theouter layer often, for example, provides good gloss and printability,the inner layer provides easy sealing and the ability to spread anddisguise condensed water. A cheap polymer as the central component canprovide the strength and handling properties, while the barrierproperties are the sum of the performance of the individual layers.) Analternative Is to use an existing standard barrier pack with a (say) 1cm² opening covered by a stuck or ultrasonically welded piece or patchof bicomponent film containing the temperature-dependent hole. The holecould be pecked at this stage (rather than earlier), e.g. mechanicallyor by laser cutters. The patches could be dispensed from a tear-offroll. This would use less of the specialist polymer and would lenditself to further elaboration, e.g. a cage to protect the curl of theradial segments or a protective layer peeled off before use. Pre-peckedstick-on patches are illustrated later. A useful step would be tocombine the patch with the package label (showing the normal informationsuch as contents, weight, `sell by` date, bar code and price) to ensurethat the correct grade of patch is applied on a pack. For its ownprotection and efficacy, the curl is preferably inwards into the pack.

In laminar flow through capillaries, the mass flow rate is proportionalto the gas density divided by the viscosity (which changes by about 10%over the temperature range 5°-20° C.). However the mass flow rate isalso proportional to the fourth power of the radius of the flow path.Using differential co-efficients of expansion of 1×10⁻⁴ ° C. (e.g.nylon/polyethylene), it should be possible to achieve the desirablefourfold increase of gas flow over the 15° C. temperature range.

Bilayers can alternatively be made using a multiple extruder. Withappropriate choice of materials and fabrication conditions, such amaterial can surprisingly be induced to lie flat at some convenienttemperature other than the temperature of fabrication. Normally howeverthe bilayer would be assembled and cut, flat, at the temperature whereminimum gas permeability is wanted. This temperature is called thelie-flat temperature. For (for example) tropical whole fruit such asbananas and mangoes, this would be 10°-15° C., with the gas flowincreasing up to say 25° C. but not opening in reverse (see later) below10° C. For vegetables, the respective temperatures would be 2°-5° C. and20° C.; accidental freezing is unlikely, and a reverse-opening preventercan sometimes be dispensed with.

Turning to FIG. 3, a selection of alternative shapes of hole isillustrated, such as V, H, S and C. They all have in common that theyare not straight and therefore, when the material curls, opendisproportionately, as is desired, but to different extents asgeometrically determined by the height/width ratio of the H, the vertexangle of V, etc. This gives a choice of hole size/temperature profiles,from which the most suitable for a particular application may beselected. The holes may be mechanically cut by a press knife mounted ina punch or may be formed by laser (e.g. excimer laser ablation), bywhich it is less likely than with mechanical cutting that the hole willhave ragged cuts that would tangle and interfere with each other whensupposed to open. In practice, mechanical cutting is adequate except foroblique cuts, described later. Some parts of the hole may be cut outmore extensively, e.g. the crossbar of the H, if the hole is required toremain of at least a certain size whatever the temperature. With lasercutting, the hole at the lie-flat temperature can have effectively zerosize. Any residual gas transmission which may be required for aparticular application at the lie-flat temperature in a material wherethe passages are then of zero size may be achieved if the materialitself has an appropriate inherent permeability.

FIG. 4 shows in cross-section the V hole of FIG. 3. It will be seen thatthe cut is oblique at the vertex of the V. (Oblique cuts are most easilymade by laser). Consider a temperature excursion below the set point ofthe laminate, which tends then to curl in the direction of the arrow.The oblique cut prevents it from opening. Another way of preventing thehole from opening during downward temperature excursions is to provide abacking flap (which may be selective as between O₂ and CO₂ or othergases) shown in chain-dotted lines on FIG. 4. Possible materials for thebacking flap include a fine polymeric net, a mesh, an open-structurednon-woven polymeric fibre fabric or an open weave woven polymeric fibrefabric. It may be seamed to the material along one edge as shown in FIG.4 or along any fraction(s) or all of its perimeter. Sometimes it will bedesirable to prevent biological contamination through the material. Insuch a case, the backing flap would be seamed all round and could be ofa microporous sheet or laminate with pore size <5 μm, for example.

FIGS. 5 and 6 show two versions of a pre-pecked stick-on patch forapplying to a 1 cm² hole in a product package.

In FIG. 5, a bilayer material according to the invention consists of 36micron polyethylene 1 stuck via an acrylic adhesive 2 (8 microns thickwhen dry) to a 12 micron polyethylene terephthalate layer 3. A passage,H-shaped in plan, was previously formed at 4 by a press knife. Thematerial is mounted via a layer 5 of pressure-sensitive adhesive on apeel-off strip of paper 6. The layer 5 is applied in such a way as toavoid the passage 4 and therefore not to impede its opening. As thetemperature rises, the material will tend to curl as shown, into thepeel-off paper, and thus the edges round the hole tend to be protectedby the peel-off paper 6. In use, a vegetable package or the like,completely wrapped but for a 1 cm² hole, is placed with the hole underthe bilayer material. The paper 6 is removed and the material applied(in the direction of the arrows) about the hole, sticking via theexposed adhesive 5. A bar code 7 and/or other information is printed(before or afterwards), and the package is ready for sale or fordistribution in the catering trade.

In FIG. 6, the same reference numerals are used for the same parts as inFIG. 5, and the mode of use is also the same. However, the polyethylene1 of FIG. 6 is not printed, but on its upper side is applied a layer ofadhesive 8, in the same pattern (avoiding the passage 4) as the adhesive5. The adhesive 8 is used to receive a continuous microporous film 9,itself printed with the bar code 7.

The microporous film 9 has a two-fold function. It inhibits the passage4 in the bilayer material from opening upwards (as drawn) in the eventof temperature excursions below the lie-flat temperature, and it reducespossible ingress of dirt and micro-organisms into the package. The poredensity and pore size of the film 9 would be selected by the designeraccording to the diffusivity and the smallest likely contaminatingorganism appropriate to the produce being packaged.

In certain applications, it will not matter if the holes enlarge belowthe lie-flat (set point) temperature, in which case neither expedient(oblique cuts, backing flaps or films) need be used. That is, on coolingto around -2° C., many respiring products meet trouble because theyfreeze and irreparably damage their cell walls (→mushy when thawed). Theenlarging of apertures as temperature falls is but a minor irritant toan already spoilt product. On the other hand, many other respiringproducts can be put into deep-freeze storage without blanching (neededsometimes to deactivate enzymes slightly active even at -20° C.); forthose, even at -20° C. a low-O₂ atmosphere is desirable for very longterm storage and thus, for those, the oblique cut or backing flap orother expedient should be used to prevent the holes from opening belowthe set point. It would be conceivable for some applications to useoblique cuts or backing flaps to allow a hole to open during downwardtemperature excursions and not upwards excursions.

It will be readily appreciated that, with no or minimal modification,these materials can be used for medical dressings (e.g. for burns),ventilation control other than for foodstuffs, (if metal-filled)temperature-dependent radiation absorbers or reflectors, dosage release(e.g. of deodorant), variable vapour barriers (e.g. vapour transmissioncontrol in shoes and clothes), thermal valves such as for appropriatelypermeable boil-in-bag or microwavable sachets, and as the female fabricof a two-fabric temperature-dependent attachment system. Anotherpossibility is that on temperature change, the opening holes can reveala backing colour or message appropriate to the temperature (e.g."Frost", on a roadside sign exploiting negative temperature excursionsfrom a 0° C. lie-flat temperature and with prevention of opening astemperature rises above 0° C. The FIGS. 1 to 4 version may be used as atemperature-dependent friction material.

Other lie-flat temperatures (i.e. slits lying flat, apertures at theirminimum opening) may be useful in various applications, e.g. -5° C. Avery high lie-flat temperature, e.g. 30° C. or 40° C., could be used forperforated incident-sunshine-activated sunblinds or tropical shading,the slits opening only on downward temperature excursions and inhibitedon upward temperature excursions.

We claim:
 1. A wrappable packaging material sufficiently flexible foruse as a wrapping and asymmetrically laminated from at least two plasticlayers disposed adjacent to each other and having different coefficientsof thermal expansion, the material having a non-straight slit, whereinthe slit forms a passage through the plastic layers, which passagevaries in size with temperature, at least over a certain temperaturerange, at a range greater than the thermal coefficient of expansion ofthe plastic layers.
 2. A wrappable packaging material according to claim1, wherein the slit has a form of a plurality of slots radiating from ajunction.
 3. A wrappable packaging material according to claim 2,wherein the junction is an aperture.
 4. A wrappable packaging materialaccording to claim 2, wherein an angle between at least one pair ofadjacent slots is at most 90°.
 5. A wrappable packaging materialaccording to claim 1, whose gas permeability increases at leastthreefold form 5° C. to 20° C.
 6. A wrappable packaging materialaccording to claim 1, wherein a temperature at which the material tendsto lie flat is in a range 0° C. to 20° C.
 7. A wrappable packagingmaterial according to claim 1, wherein the slit is formed by a laser. 8.A wrappable packaging material according to claim 1, wherein the layersare dimensionally inert to moisture.
 9. A wrappable packaging materialaccording to claim 8, wherein at least one of the layers is hydrophobic.10. A wrappable packaging material according to claim 1, wherein a firstof the layers is selected from the group consisting of polyester,polyamide, cellulose, and polyethylene terephthalate.
 11. A wrappablepackaging material according to claim 1, wherein a second of the layersis selected from the group consisting of polyolefin and ethyl vinylacetate plastic.
 12. A wrappable packaging material according to claim1, with means to inhibit passage through the slit when a temperaturedeviates in one sense from a reference temperature.
 13. A wrappablepackaging material according to claim 12, wherein said means comprises abacking flap.
 14. A wrappable packaging material according to claim 12,wherein said means consists in that the slit is obliquely cut.
 15. Awrappable packaging material according to claim 1, wherein thicknessesof at least some of the layers have a thickness in at least approximateproportion to an inverse cube root of their respective Young's moduli.16. A wrapper packaging material according to claim 1, furthercomprising a metal foil layer in contacting relation with one of theplastic layers.
 17. A container comprising the wrappable packagingmaterial according to claim
 1. 18. A method of packaging edible produce,comprising the step of wrapping a portion of the produce in thewrappable packaging material according to claim
 1. 19. A containeraccording to claim 17, wherein the wrappable packaging material includesmeans for containing produce.
 20. A container according to claim 17,wherein the wrappable packaging material is microwaveable.
 21. Awrappable packaging material according to claim 11, wherein thepolyolefin layer is metalized.
 22. A method of packaging edible produceaccording to claim 18, wherein the entirety of the produce is wrapped insaid wrappable packaging material.
 23. A container according to claim17, wherein the wrappable packaging material is boilable.
 24. Awrappable packaging material according to claim 1, wherein the layersare substantially transparent.
 25. A packaging material asymmetricallylaminated from at least two plastic layers disposed adjacent to eachother and having different coefficients of thermal expansion, thematerial having a non-straight slit, wherein thicknesses of the layersare in at least approximate proportion to an inverse cube root of theirrespective Young's moduli, and wherein the slit forms a passage throughthe plastic layers, which passage varies in size with temperature, atleast over a certain temperature range, at a rate greater than thethermal coefficients of expansion of the plastic layers.